In the production of electrical insulations, for example, in the production of high voltage applications, epoxy resin compositions comprising a hardener component, such as an acid anhydride hardener component, can be used due to their excellent electrical and mechanical properties. However, using acid anhydrides may cause health damage, for example, when such compounds are industrially used in open processes, such as in open impregnation or wet winding applications.
In order to minimize such health damage, it has been proposed to use epoxy resin compositions which are free of compounds which generate emission of highly volatile organic compounds during processing, i.e., which are volatile-free, for example, which are free of acid anhydrides and volatile diluents such as styrene or methyl methacrylate, and which are cured in the presence of a catalyst. Such epoxy resin compositions contain a latent catalyst, the latent catalyst comprising, for example, a metal acetylacetonate or a mixture of such compounds. The term latent catalyst means that the catalyst is present as an integral part within the composition.
For electrical insulation applications, for example, for high voltage applications, however, it can be beneficial to fulfill requirements for material properties and processing parameters. In case of impregnation applications, for example, for impregnating mica tape wound coils for electrical machines or for the impregnation of paper wound conductors for bushings, or for filament wet winding applications, it is substantial that the curable epoxy resin composition has a long pot life, i.e., slow curing speed at processing temperature and a short gel time, i.e., fast cross-linking reaction, for example, polymerization reaction, at curing temperature. Further, a low dielectric loss of the final cured insulating material within a wide temperature range can be beneficial, for example, for high voltage applications. However, the properties of a long pot life and a short gel time are contradictory. In general, a long pot life goes along with a prolonged gel time, caused by the low reactivity and slow polymerization speed of the composition, while a short gel time goes along with a short pot life, caused by the elevated reactivity and elevated polymerization speed of the composition.
For most impregnation applications, such as impregnating or filament wet winding applications, a low viscosity can be desirable for proper processing. In the absence of any hardener component or volatile diluents, the epoxy resin needs to be heated up for decreasing the viscosity. This heating up to an elevated temperature, however, causes a viscosity increase and a shortened pot life. For vacuum pressure impregnation (VPI) of mica tape wound coils and wet winding processes for fibers, the resin in the tank or basin is used for several production runs and products. Thus, a long pot life at processing temperature with stable low viscosity is substantial for obtaining good impregnation quality and keeping production costs low as, for example, vacuum pressure impregnation (VPI) and filament wet winding processes are continuous production processes using partially open tanks or basins.
Fast gelling in the curing oven, i.e., after impregnation or winding, can be important in order to avoid the curable epoxy resin composition dripping off the impregnated or the wet wound parts before being cured. Thus, short gel times below 10-30 minutes at curing temperature can be required.
Epoxy resin formulations comprising an epoxy resin component and a catalyst system composed of a metal acetylacetonate and a phenolic compound are disclosed, for example, in GB 1402899. Such catalytic systems are described as providing stability to the curable epoxy resin formulation at room temperature for a long period of time. Basically, the chemical activity of such catalytic systems is not limited to the type of epoxy resin. GB 1402899 describes the activity of the catalytic system at elevated temperatures, such as 100° C. to 160° C., while using epoxy resin compositions, especially cycloaliphatic compounds, which have a low viscosity at room temperature. Such low viscosity at room temperature allows use of these cycloaliphatic compounds in VPI and filament wet winding processes at room temperature. Also, storage of said curable epoxy resin formulations was performed at room temperature whereby no gelation occurred. However, for producing electrical insulators from aromatic epoxy resin compounds, such as from diglycidylether of bisphenol A (DGEBA), due to the high viscosity of DGEBA at room temperature, it is beneficial to keep the curable epoxy resin formulation at elevated temperature during processing, such as at about 50° C., for a longer period of time without gelling.